Arteriosclerosis is one of the lifestyle-related diseases in which excessive lifestyle habits are risk factors. Arteriosclerosis advances from youth with very few symptoms, and if left untreated, arteriosclerosis may develop into cerebral infarction, cardiac infarction, angina pectoris, and the like. It is difficult to cure arteriosclerosis. Therefore, it is important to find symptoms of arteriosclerosis at an early stage and prompt the patients to improve lifestyle habits to slow down the progression of the diseases.
There are various diagnostic methods for examining arteriosclerosis. Among the methods, ultrasonic echo examination allows noninvasive direct observation of blood vessels and has been gaining a foothold as an indispensable examination in the current clinical diagnosis. The ultrasonic echo examination for arteriosclerosis diagnosis evaluates the presence of fat called plaque adhered inside blood vessels and intima-media complex thickness (IMT). However, at an early stage of arteriosclerosis, there is no plaque, and the IMT is thin and hard to measure. It is difficult to find symptoms at an early stage in such a morphological observation.
As contrasted with this, since arteriosclerosis is sclerosis of artery and a loss of elasticity, there is an idea that movements of artery associated with heartbeats can be evaluated. Unlike the morphological measurements such as plaque and IMT measurements, the aid of computer is inevitable when performing a quantitative measurement of the movements of artery.
Patent Document 1 discloses that measurement of variation in the carotid artery diameter associated with heartbeats can obtain an indicator of arteriosclerosis.
In Japanese Patent Application No. 2007-239198, the present applicant proposes a method of using an ultrasound B-mode major axis cross-sectional image to estimate variation in the carotid artery diameter associated with heartbeats. The method is executed based on a precondition that the carotid arteries are homogenous regardless of the positions on the major axis (y axis) and show equivalent variations according to heartbeats. Although satisfied in many cases, the precondition is not satisfied in a case where there is local arteriosclerosis in the carotid artery. Therefore, in such a case, the method previously proposed by the present applicant cannot be applied. The major axis of a carotid artery is an axis extending in an extension direction of the carotid artery, and an axis orthogonal to the major axis is referred to as a minor axis of the carotid artery.
In a case where there is local arteriosclerosis in a carotid artery, a minor axis cross-sectional image of the carotid artery, which is an ultrasound B-mode image, can be used to estimate variation in a carotid artery associated with heartbeats. However, the size of one pixel of the ultrasound B-mode image is about 0.07 to 0.11 mm, and the resolution is sufficiently high. Therefore, a technique for capturing the variation in a carotid artery with accuracy finer than the size of one pixel is indispensable.
In a method proposed by the present application in Japanese Patent Application No. 2007-239198, information related to a spatial shape of a carotid artery major axis cross-sectional image is used to improve the estimation accuracy. However, when a minor axis cross-sectional image of a carotid artery is used, it is difficult to use information related to a spatial shape of the image to improve the estimation accuracy due to the following reasons. The first reason is that although the minor axis cross section shapes of a carotid artery are substantially circular, individual differences are large. Therefore, it is difficult to express the shapes uniformly. The second reason is that the area occupied by the carotid artery in an ultrasound B-mode image is smaller in a minor axis cross-sectional image than in a major axis cross-sectional image. Therefore, usable information is scarce.
Non-Patent Document 1 proposes a method of manually designating a point on a carotid artery wall on a B-mode image, and then using an RF signal to estimate and track the movement of the designated point on the carotid artery wall in 10 μm accuracy. The method is in practical use.
When an RF signal is used as in the method proposed in Non-Patent Document 1, the estimation accuracy can be improved using a phase difference tracking method with an increased sampling rate. However, such a method for improving the estimation accuracy cannot be applied to a method of using a B-mode image to estimate and track the movement of a carotid artery. Furthermore, when the movement of a carotid artery is tracked, it is advantageous to use an RF signal from the viewpoint of accuracy. However, ultrasonic diagnostic equipment capable of outputting RF signals is limited to only some models. The ultrasonic diagnostic equipment therefore cannot be regarded as being in common use and lacks versatility.    Patent Document 1: Japanese Patent No. 3882084    Non-Patent Document 1: Harada A, “Measurement of Arteriosclerosis Indicator by Echo-Tracking Method (in Japanese)”, Japanese Journal of Medical Ultrasound Technique, Vol. 28(3): pp. 29-35, 2003.    Non-Patent Document 2: G. H. Granlund, “Fourier preprocessing for hand print character recognition”, IEEE Trans. on Computers, Vol. C-21, pp. 195-201, 1972.    Non-Patent Document 3: Chris L. de Korte et al, “Characterization of Plaque Components with Intravascular Ultrasound Elastgraphy in Human Femoral and Coronary Arteries In Vitro”.    Non-Patent Document 4: Radj A. Baldewsing, Chris L. de Korte et al. “Comparison of Finite Elements Model Elastograms and IVUS Elastograms acquired from Phantoms and Arteries”, IEEE Symposium, pp. 1873-1875, 2002.    Non-Patent Document 5: Hayashi K, “Anatomy of Japanese and Japanese Medical Experimental Animals and Physiological Modulus (in Japanese)”, Second Edition (corrected reprint edition), Bunkyo Shoin, pp. 9, 1965.